1. Field of the Invention
The present invention relates generally to voltage sensing apparatus and more particularly to a bushing assembly that includes an integral capacitance screen.
2. Description of Related Art
Various apparatus have been proposed for use in making electrical connections and for sensing electrical parameters such as voltage and current in medium-voltage electrical power distribution systems. For example, this apparatus is useful to provide electrical connections and also to provide data for automated distribution systems, phasing information, troubleshooting of faulted cables, etc. Some of these arrangements utilize separable insulated connectors on power cables while others provide receiving passages through which the energized electrical cables are passed, e.g. see ANSI/IEEE Standard 386. Still other arrangements provide coupling/sensor assemblies that interfit with the conventional test point caps of elbows for cable terminations. Some of these devices utilize transformers, pick-up coils, resistive dividers and capacitive coupling to sense circuit parameters. One device, available from Lindsey Manufacturing Company of Azusa, Calif. as the Elbow Sense (trademark) Voltage Monitoring Plug, utilizes a voltage monitoring plug that replaces the standard plug on 600 ampere elbow assemblies, commonly referred to as xe2x80x9cT-bodyxe2x80x9d elbows. This arrangement utilizes a precision resistive voltage divider to provide an output proportional to line-to-ground voltage. Another device available from Lindsey Manufacturing Co. is a VSB Voltage Sensing Bushing that utilizes a capacitive voltage divider. This interface bushing device is arranged to be directly affixed to a switchgear tank or the like, replacing the existing bushing that interfits with the cable-terminating elbow devices such as the 600 ampere T-body components.
Devices which capacitively couple to the test point of an elbow connector are exemplified by U.S. Pat. Nos. 4,814,933, 5,077,520 and 5,095,265.
Arrangements which utilize electrodes spaced from a cable or conductor are shown in U.S. Pat. Nos. 3,970,932, 3,991,367, 4,074,193, 4,823,022, 5,051,733, 5,521,567, 5,726,390 and 6,031,368. The bushing with voltage tap of the aforementioned U.S. Pat. No. 5,726,390 utilizes an annular plastic member with a layer of conductive lacquer. A connector extends from a leg of the annular member to the exterior of the bushing.
Another type of voltage sensor, shown in U.S. Pat. No. 4,002,976, utilizes a capacitor having one end directly in contact with a high voltage terminal and a step-down transformer in series with the capacitor, the output of the secondary of the step-down transformer providing a voltage proportional to the terminal voltage.
While these various arrangements are generally useful, it would be desirable to provide a voltage sensing bushing that is economical to manufacture, exhibits improved strength and is less susceptible to environmental ingress.
Accordingly, it is a principal object of the present invention to provide a voltage sensing apparatus with an integral capacitance screen having a predetermined pattern of openings.
It is another object of the present invention to provide a voltage sensing apparatus that is formed via a molding process about a polymeric capacitance screen having a generally open structure.
It is a further object of the present invention to provide a capacitance screen that is molded from a conductive polymer and including a predetermined pattern of openings.
These and other objects of the present invention are efficiently achieved by a voltage sensing apparatus including a voltage tap point that is useful in representing the voltage on a conductor that passes through the voltage sensing apparatus. The voltage sensing apparatus also performs the function of a separable insulated conductor or the like, e.g. a conventional bushing or bushing insert as utilized in the electrical power distribution field. The voltage sensing apparatus includes a molded body having an embedded capacitance screen that provides the output. The body is molded about the capacitance screen. An arrangement is provided for locating the capacitance screen within the molded body. In a preferred embodiment, the capacitance screen is molded from a conductive plastic preferably formed with an open mesh or screen structure that permits the free flow of molding material.